While a restore to 6.1.2 proper (via iTunes and all that) isn't possible at the moment, a program like iLEX R.A.T. or Semi-Restore might do the trick. They'll revert your phone to a blank but freshly-jailbroken slate--the phone will have no customizations or data on it, but will have Cydia installed.

Please be aware that if you have any damage to your system or core files, these tools cannot repair them and your issue may persist. At this point, a restore (the bad, non-jailbreakable kind) to 6.1.4 may be required.